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Lysocline, Calcium Carbonate Compensation Depth, and Calcareous Sediments in the North Pacific Ocean!

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Lysocline, Calcium Carbonate Compensation Depth, and Calcareous Sediments in the North Pacific Ocean! Pacific Science (1988), vol. 42, nos. 3-4 © 1988 by the University of Hawaii Press. All rights reserved Lysocline, Calcium Carbonate Compensation Depth, and Calcareous Sediments in the North Pacific Ocean! C. T. A. CHEN,2 R. A. FEELy,3 AND J. F. GENDRON3 ABSTRACT: An extensive oceanographic investigation has been carried out in the North Pacific Ocean. The purpose of this report is to present the results of two cruises in which we participated and to report additional carbonate data from samples collected for us in the North Pacific. These data are combined with data from the literature to provide an overall picture ofthe carbonate system in the North Pacific. The degree ofsaturation ofseawater with respect to calcite and aragonite was calculated from all available data sets. Four selected cross sections, three longi­ tudinal and one latitudinal, and two three-dimensional graphs show that a large volume of the North Pacific is undersaturated with respect to CaC03. The saturation horizon generally shows a shoaling from west to east and from south to north in the North Pacific Ocean. It was found that the lysocline is at a depth much deeper (about 2500 m deeper) than the saturation horizon of calcite, and several hundred meters shallower than the calcium carbonate compensation depth. Our results appear to support the kinetic point of view on the CaC03 dissolution mechanisms. Differences in the abundance of the calcareous sedi­ ments are explained by differences in the calcium carbonate compensation depth. THE BEHAVIOR OF THE carbonate system in report additional carbonate data from sam­ seawater is one of the most complex topics in ples collected for us in the North Pacific. oceanography. The system has long interested These data are combined with data from the many oceanographers from various fields be­ literature to provide an overall picture of the cause it plays an important role in all three carbonate system in the North Pacific. Details subspheres of the earth (biosphere , lithosphere, on data collection are presented elsewhere and hydrosphere). (Chen et al. 1986) and are not repeated here. An extensive oceanographic investigation has been carried out in the North Pacific Ocean . The purpose of this paper is to present DEGREE OF SATURATION OF CaC03, the results of two cruises in which we par­ LYSOCLINE, AND CALCIUM CARBONATE ticipated (Chen 1982, Chen et al. 1986) and to COMPENSAnON DEPTH It is well known that deep waters of the 1 This work was supported by the National Science Council of the Republic of China (grant NSC 76-0209 M North Pacific Ocean are the oldest in the 110-03), U.S. Department of Energy (grant 19X-89608C), world oceans. Hence, the concentration of car­ the Atmospheric Research Laboratory of the U.S. Na­ bonate ion in the North Pacific Ocean shows tional Oceanic and Atmospheric Administration (NOAA) the lowest value. Takahashi et al. (1981) and the National Science Foundation (grant OCE­ CO~ - 8502474). Contribution No. 974 from NOAA /Pacific found a low mean concentration in the Marine Environmental Laboratory. Manuscript accepted Pacific Ocean and attributed it to the low 23 July 1987. alkalinity/total CO2 ratio of this area. The 2 Institute of Marine Geology, National Sun Yat-Sen result of low CO~ - concentration is a lower University, Kaohsiung, Taiwan, R.O.C. degree ofsaturation and shallower calcite and 3 Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, 7600 Sand aragonite saturation horizons in the Pacific Point Way NE, Seattle, Washington 98115-0070. Ocean than in other oceanic regions. 237 238 PACIFIC SCIENCE, Volume 42, July/October 1988 The distribution of the degree of satura­ Berner (1976), respectively, are used. In order tion of seawater with respect to calcite and to obtain the apparent solubility product at in aragonite is presented below . The relation­ situ conditions, the pressure effects on the ships between the saturation state and the solubility products summarized by Culberson calcium carbonate compensation depth and (1972) are used. the lysocline are also addressed. DEGRE E OF SATURATION OF TH E METHOD OF CALCULATING THE DEGREE OF SURFA CE WATER SATURATION Figures I a and 1b show the correlation In calculating the degree ofsaturation ofsea­ of temperature with the degree of saturation water with respect to calcite and aragonite, we of sea~ater with respect to calcite and need to calculate the carbonate ion concentra­ (nJ aragomte (n ), respectively, for the surface tion from the measured carbonate data. The a waters of the North Pacific Ocean. It is evi­ CO~- concentration in seawater at in situ d~nt that all surface waters are supersaturated temperature and pressure conditions has been WithCaC0 (Alekin and Katunin 1973). Lin­ computed from the NOAA Eastern North 3 ear correlations between , and tempera­ Pacific CO Dynamics Cruise, 1981 (ENP), nc na, 2 ture are found in the data sets for both fig­ and. Western North Pacific CO Dynamics 2 ures. A high degree of saturation is observed CrUl~e ,. 1982 (WNP), salinity, temperature, in high-temperature Pacific surface seawater alkalinity, pH, and calcium data. In addition 0 (Lyakhin 1968) for calcium carbonate. The to our two meridional sections along 165 E 0 degree of saturation is strongly influenced by and 150 W, all GEOSECS (Geochemical the titration alkalinity (TA) versus total CO Ocean Section Studies, 1973-1974[GS]) and (TC0 ratio, which also shows a linear de: INDOPAC (Scripps Institute of Oceanogra­ 2) pendence on surface temperature as suggested phy, 1976 [IP]) alkalinity and total CO data 2 by Feely et al. (1984). from the North Pacific Ocean are used to The isopleths of the surface saturation val­ calculate the degree ofsaturation ofseawater. ues are shown in Figures 2a and 2b for calcite Two cross sections, one longitudinal along ~r~go.nite, roughly 1800 Wand one latitudinal along and respectivel y. The overall pat­ 0 tern is Similar to the temperature distribution roughly 35 N, are selected from GEOSECS which in turn is influenced by the surface stations to demonstrate the distribution of oceanographic circulation and other factors. saturation values. The effects of pressure on The n and n decrease slightly from west to the dissociation constants for carbonic and c a east. In the western subarctic regions low boric acids determined by Culberson et al. n and are also found in the cold 'Gulf of (1967) and Culberson (1972) are used for the n, Alaska. A sharp change occurs from north computation. Our expression for the satura­ to south at the region of mixing between the tion states for calcite and aragonite in sea­ cold Oyashio and the warm Kuroshio around water is in percent of saturation: 0 40 N . A poorly defined area of high nc and ICP n values occurs near the center of the sub­ n= - - x 100% a K~p tropical gyre west of Hawaii. Figures 2a and 2b represent mainly summer where ICP (ion concentration product) = conditions, and the contours are likely to [Ca2+] x . [ ~O ~ - ] (M jkg)2 and K~p = appar­ move southward in winter as surface tempera­ ent solubility product for calcite or aragonite. ture decreases. For instance, Lyakhin (1970) The apparent solubility products for calcite reported 300-400% saturation of calcite in and aragonite in seawater at 1 atm total pres­ the Sea of Okhotsk in summer but less than sure determined by Ingle et al. (1973) and by 150% saturation in winter. Lysocline, Calcium Carbonate Compensatio n Depth, and Sediments- CHEN, F EELY, AND G ENDRON 239 T,OC 5 10 15 20 25 200 • ENP o WNP o • I P A GS 300 "" Oc a a aa a lllx>C\ 400 a a a a.t a a a a a " o a a " " 500 " .. " 600 " a T, °C 5 10 15 20 25 • ENP o o WNP a IP A GS 200 .. 300 e I a A " .. A 0 400 " .A b AA F IGURE 1. Corre lation of surface (a) nc and (b) n. with temperature in the North Pacific. The lines are rough fits by eye. 240 PACIFIC SCIENCE, Volume 42, July/October 1988 N - , , ,.... 0 • o o • • 0 0 300 o 0 • ... .... ... ~ ... ... ... ... ... ... • 0 0 • • o o ~ , - -400- , o /' I / 0/ • > 400 I~ { 0 / \ / o ... / 10 0/0 " -, ., 10 ...... ..._-- • • o o b o 140 E 160 180 160 140 120W FIGURE 2. Distr ibution of sur face (a) n, and (b) n. in the Nor th Pacific Ocean. Areas enclosed by dashed circles probabl y have the highest degree of supersaturation. Lysocline, Calcium Carbonate Compensation Depth, and Sediments-CHEN , FEELY, AND GENDRON 241 VERTICAL DISTRIBUTION OF DEGREE OF Discoverer cruise (Betzer et al. 1984). Their SATURATION findings revealed that the depth at which a high dissolution rate of aragonite was found Four cross sections of saturation values is shallower at high latitudes than at mid­ of calcite and aragonite are plotted from the latitudes. These findings support our satura­ ENP, WNP, and GEOSECS data. tion profile, which shows a concave structure at mid-latitudes. A similar structure was re­ 1. The 35° N Cross Section ported by Betzer et al. (1984). A sharp change from undersaturation to The latitudinal cross section along approx. supersaturation for calcite is found between 35° N was selected also by Takahashi (1975), stations WNP5 and WNP6. The oversatur­ who used TC02 data unadjusted for the fossil ated water at WNP3 and WNP5 may be the fuel CO 2 input to calculate saturation values; result of the younger age of the seawater in and only nc was calculated in his paper. The that region . Station WNP6 defined the south­ revised carbonate data (Takahashi et aI. 1980) ern limit of North Pacific deep water that has are used to recalculate the degree of saturation low oxygen and high nutrients.
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